1. Field of the Invention
The present invention relates to a printing mechanism and more particularly to a printing mechanism for use in a data display apparatus comprising a type wheel supported on a driving shaft and carrying thereon characters and symbols to be printed.
2. Description of the Prior Art
Various types of printing mechanisms have been proposed. One type of printing mechanism which has been known and used is provided with one, two or more type wheels arranged in parallel and movable toward and away from a recording medium. The type wheel is moved toward the recording medium by a control apparatus for selecting and controlling the point in time for printing. Printing is effected by this swing motion of the type wheel. An example of such a type of printer having a swing movable type wheel(s) is disclosed in U.S. Pat. No. 3,807,301.
However, the arrangement disclosed in the above mentioned patent has some problems. In the arrangement, the type wheel and the type ring are connected by an elastic member, and the type ring is pushed toward the recording medium by a pawl. When the pawl is disengaged after printing, the type wheel is moved back by the elastic member in the same direction as the type ring. Therefore, the type on the surface of the recording medium attempts to move back while being rotated together with the type ring. This rotation of the type results in rubbing of the recording medium surface against the type and thus the printed character is rubbed and the trouble of so-called ghost is caused. This is particularly true in the case wherein the type wheel and types are formed of elastic material, such as rubber, and ink is applied to the types by an ink roller which is generally easy to operate and assures clear and sharp printing.
Also, in the known mechanism, the spring is continuously tensioned during printing and the pressure applied to the type under printing increases in proportion to the length of holding time by the pawl. Therefore, the intensity of printed characters on the recording medium is greatly affected by variation of the length of printing time. Furthermore, at the end of every printing, the pawl must be disengaged against the tension force of the spring. Accordingly, it is required to use a strong spring for returning the pawl to its home position. Consequently, a large electromagnet is required to drive the pawl, and therefore a large amount of electric power is required. For these reasons, it has been difficult to create a printer which is simple in structure and low in power consumption.